What exactly is it in the Nikon zoom lens that makes it so much better than the Tamron?

Before buying my Tamron lens for my Nikon D -40, I asked in this forum if Nikon was noticeably better than Tamron. I got two answers both of which said they were "okay" with Tamron. That should have been the tip off. My Tamron 18-200mm zoom lens takes very good photos but they don't come out crytal clear the way the Nikon lens can shoot them ( I borrowed a friend's zoom lens later).So my question: what exactly makes the difference in the Nikon, obviously the glass but much more must be involved.

The build quality and material quality are the two big differences. If the overall quality was as good as Nikon/Canon/etc, they would be charging almost the same price. Plus, manufacturers don't give away all of their proprietary information as to how the system works (otherwise it wouldn't be proprietary).

Third party manufacturers normally "reverse engineer" systems or look at schematics to design their own parts. Sometimes the manufacturer will give basic info, such as "this contact supplies this voltage", or "this contact reads ___ information".

When I first started in digital, I bought Tammy lenses due to the savings. I have never had great luck with them though (clarity issues, slow focusing, sticking zoom/focus, etc.). Now when I want a new lens, I just save up until I can afford one from the camera manufacturer.

Since a Nikon 18-200mm will be THREE TIMES THE COST what do you expect?

In two words… The glass and construction. I own one Tamron lens, the one you're talking about the
18-200.It has focus and aberration issues. When I use it it must be in bright light and I always run it at f/8 and let the camera decide the shutter speed. Based on what I know, owning this lens,
I won't buy another Tamron.

I'm a Canon user and I can tell you, at least in my experience, it's the glass and construction.

I have to admit that I've shot with (and continue to use) 3rd party lenses when the budget won't let me get OEM Nikon optics.

One of my workhorses is my Nikon 18-200mm AF-S VR. Though I bought it the first month it came out, and at full retail, I still love it. When I was touring China in 2007, it was the ONLY lens I took.

That being said, the big difference between OEM and 3rd party lenses (besides the obvious price difference) is that 3rd party lenses are reverse-engineered. That means that Tamron buys Nikon lenses, and tries to figure out what each mechanical and electronic connection does.

Most of the time, that flattering imitation is invisible. Everything works as it should. BUT, when the camera maker comes out with newer models or accessories (flashes, etc) they cleverly make sure that their original lenses work with all future models. 3rd party lens makers can't guarantee that.

Sigma, Tamron, Tokina, and the others will either charge you to rechip their lens, or regretfully tell you that the old model is incompatible, and you will need to purchase a newer version.

Better optical design and quality control makes the difference when it comes to image quality. However, this is expected as Nikon lenses are usually much more expensive than comparable Tamron lenses. Tamron is a lens manufacturer that mainly appeals to beginners to enthusiasts looking for a cheaper alternative to Nikon lenses.